It's simply because it was a POPULARITY CONTEST (when public voting is involved, it becomes a popularity contest, the voting public thought she was entertaining), the ones who have the bigger fan following will win, simple. Rubina & Rakhi has the bigger fan following of the ones who performed, hence they won.
I'll give you another example of a similar situation, on BB11 there was a singing task between Shilpa & Hina & voot viewers had to vote for whom they think sang the best. Hina obviously killed it with her singing & was the best, however Shilpa was TERRIBLE in her singing, like not even 1% close to how amazing Hina was, yet she won with a decent margin. Why? because it's a popularity contest at the end of the day, the one with the bigger fan following will win, simple.
Rubina EARNED her loyal fans from BB & hence she won, as she's the most loved contestant this season, majority of BB fans love/rooting for her. That's a FACT even her haters/critiques can't deny. ⭐️